The invention relates to a method and apparatus for sensing the passage of sheets through a nip between a pair of guide surfaces by sensing with sensing means the relative deflection between the guide surfaces in response to the presence of a sheet in the nip. Such methods and apparatus are hereinafter referred to as of the kind described.
One example of a method and apparatus of the kind described is illustrated in No. WO-A-82/01698. In this construction an expected thickness of a banknote is preset and a suitable signal is fed to an auto reference circuit which adds to the expected thickness a value representing a datum corresponding to a roller resting on a guide surface. Subsequently, when a note passes under the roller which is connected via an arm to a linear variable differential transformer, an output signal is fed to comparators which will determine whether or not a note is present. Between the passage of each note, the datum level is rechecked and a suitable correction is made by the auto reference circuit to the reference signals fed to the comparators.
One of the problems with methods and apparatus of the kind described is that the guide surfaces may not be truely uniform and indeed may vary during use. This is particularly the case where one of the surfaces is formed in a resilient material such as rubber. One of the main difficulties with this is that the degree of variation is often of the same order of magnitude as the thickness of a sheet such as a banknote. Furthermore the apparatus can be affected by dust.